Research progress of flexible pressure sensor based on MXene materials

Flexible pressure sensors overcome the limitations of traditional rigid sensors on the surface of the measured object, demonstrating broad application prospects in fields such as sports health and vital sign monitoring due to their excellent flexibility and comfort in contact with the body. MXene, as a two-dimensional material, possesses excellent conductivity and abundant surface functional groups. Simultaneously, MXene's unique layered structure and large specific surface area offer a wealth of possibilities for preparing sensing elements in combination with other materials. This article reviews the preparation methods of MXene materials and their performance indicators as sensing elements, discusses the controllable preparation methods of MXene materials and the impact of their physical and chemical properties on their functions, elaborates on the pressure sensing mechanism and evaluation mechanism of MXene materials. Starting from the four specific application directions: aerogel/hydrogel, ink printing, thin film/electronic skin, and fiber fabric, we introduce the research progress of MXene flexible pressure sensors from an overall perspective. Finally, a summary and outlook for developing MXene flexible pressure sensors are provided.


Introduction
2][3][4] characterized by their exceptional exibility, high sensitivity, and lightweight, exible sensors nd extensive applications in the measurement of diverse human body information such as joint movement, sweat, and temperature.6][7][8] Regardless of their type, sensors fundamentally convert non-electrical quantities into electrical signals, thus enabling the measurement, transmission, processing, and control of different signals.][14][15][16][17][18][19] MXene is a type of two-dimensional layered nanomaterial characterized by its general chemical formula M n+1 X n T x (n = 1, 2, 3), where M represents transition metals (such as Sc, Ti, Zr, V, Nb, etc.), X represents carbon and/or nitrogen, and T x represents surface functional groups (such as -OH, ]O, -F, etc.).Among the reported sensing materials, MXene stands out for its large specic surface area and abundant functional groups on the surface.MXene has metal-like electrical conductivity, and its abundant functional groups on the surface (-OH, ]O, -F, etc.) give it good hydrophilicity and easy to form micro-nanostructures by chemical bonding with other materials.In terms of mechanical properties MXene materials have excellent exibility and stretchability, which enables MXene materials to adapt to a wide range of complex strain scenarios such as bending, stretching, or compression for reliable mechanical sensing.Aer continuous research, the tensile and compressive properties of exible pressure sensors have been further improved by changing the structure of the sensing layer material, such as bionic structure, island bridge structure, etc. 20,21 However, it remains challenging for a single material to simultaneously meet the requirements for exibility, electrical conductivity, biocompatibility, and resistance to magnetic interference which are crucial for diverse applications.Composite materials offer a solution by overcoming the limitations of individual materials and can yield exible sensors 2 Preparation and properties of MXene materials

Preparation of MXene
In 2011, Gogotsi et al. performed the rst selective etching of aluminum (Al) from aluminum titanium carbide (Ti 3 AlC 2 ) using hydrouoric acid (HF).This process allowed for the preparation of multi-layer MXene powder Ti 3 C 2 T x . 28Subsequently, researchers have synthesized various MXene materials such as Ti 2 CT x , V 2 CT x , Cr 2 CT x , Zr 2 CT x , etc., expanding the family of MXene to include transition metal carbides, nitrides, and carbon nitrides. 29The chemical equation for etching titaniumcarbide aluminum with hydrouoric acid is as follows: The layered MXene material with an organized structure was obtained by stratifying titanium-carbide aluminum through etching with hydrouoric acid.However, as hydrouoric acid is corrosive to organisms, Ghidiu et al. 30 employed the LiF/HCl mixture etching method to produce MXene.This resulted in Ti 3 C 2 T x with clay-like morphology characteristics, which could be further separated into single pieces of Ti 3 C 2 T x with submicron transverse size by ultrasonic treatment in water.Additionally, it has been reported that MXene can be efficiently stratied using tetrabutylammonium hydroxide (TBAOH) and an amine-assisted method (Fig. 2), which helps improve the yield of MXene material. 31,32[35][36]

Electrical property
The conductivity of the sensing material is a key factor affecting the sensing performance of exible sensors, and MXene has a high conductivity similar to that of metals.Lipatov et al. 37 prepared eld-effect transistors (FETs) based on a single Ti 3 C 2 T x sheet as shown in Fig. 3a and measured their electrical properties.The single Ti 3 C 2 T x sheet exhibited a high conductivity of 4600 ± 1100 S cm −1 and eld-effect electron mobility of 2.6 ± 0.7 cm 2 V −1 s −1 .
According to density-functional theory (DFT), the electrical properties of MXene are related to its elemental composition and surface groups. 38Targeted control of the electrical properties can therefore be achieved by purposefully changing the  elemental composition and/or surface terminations of MXene.For example, in Urbankowski's work, 39 Mo 2 C was used as a carbide MXene precursor, which was converted to Mo 2 N phase by ammonia decomposition (Fig. 3b).Aer heat treatment at 600 °C, Mo 2 N retained the layered structure of MXene.During subsequent electrical resistance measurements, the pristine Mo 2 CT x lms were treated under the same conditions in an inert atmosphere to eliminate defects introduced by the heat treatment.The results showed that the resistivity of the treated Mo 2 CT x lms was still an order of magnitude higher than that of the Mo 2 NT x lms.
In addition, the actual electrical properties of MXene lms are macroscopic manifestations of multilayer stacked nanosheets, and thus the intercalation between the layers also determines their electrical properties.For example, cations in reagents (tetramethylammonium ions (TMA + ), ammonium ions (NH 4 + ), and lithium ions (Li + )) and organic molecules (dimethylsulfoxide (DMSO) and isopropylamine) can be inserted into the spacing of MXene's layers, altering the electrical properties of the MXene lms. 40,41Therefore, the elemental composition, surface termination, and embedding of MXene lms can be controlled by post-processing modications to effectively achieve targeted control of their electrical properties.
It is worth noting that MXene's large specic surface area and surface-rich functional groups combine with H 2 O, O 2 , and free ions in the air when exposed to air, which inhibits the transfer of interlayer electrons and weakens the electrical conductivity of MXene, and thus the storage environment and the storage time can seriously affect the electrical conductivity of MXene materials.

Mechanical property
The mechanical properties of exible sensing materials play a crucial role in determining the exibility, detection range, and durability of exible pressure sensors, which are important parameters for evaluating exible pressure sensing materials.Guo et al. 42 investigated the mechanical properties of monolayer MXene using rst-principles calculations, taking Ti n+1 C n as an example.Experimental results demonstrate that two-dimensional Ti 2 C can endure signicant strains of 9.5%, 18%, and 17% under biaxial and uniaxial tension along the x/y directions, respectively.Aer modifying the oxygen through surface functionalization, the strain of Ti 2 CO 2 increased to 20%, 28%, and 26.5% respectively (exceeding the biaxial strain limit of 15% for graphene).
Borysiuk et al. 43 constructed an ideal Ti n+1 C n monolayer model and performed classical molecular dynamics simulations to obtain the stress/strain curve of the MXene sample under tensile load, as shown in Fig. 4a.For a strain 3 < 0.01, the Young's modulus values of Ti 2 C, Ti 3 C 2 , and Ti 4 C 3 are 597 GPa, 502 GPa, and 534 GPa, respectively.In addition to computational studies, Lipatov et al. 44 conducted atomic force microscopy (AFM) nanoindentation experiments to measure the mechanical properties of MXene.They utilized a tip with a radius of 7 nm and obtained the force-displacement curve.The Young's modulus of single-layer Ti 3 C 2 T x was determined to be 0.33 ± 0.03 TPa (Fig. 4b).Therefore, MXene is an excellent choice for applications requiring nanodevices and composites with high mechanical property demands.Although monolayer MXene exhibits a broad theoretical strain range and high Young's modulus, in pressure sensor applications, MXene is typically composed of laminated nanosheets.This introduces certain disparities between the theoretical and actual mechanical properties.In the presence of an external load, cracks tend to emerge between the nanosheets rather than within the sheets themselves, as the binding forces between the nanosheets are predominantly governed by van der Waals interactions.Strengthening these interface interactions becomes essential.Consequently, during the fabrication of MXene sensing materials, various compounds such as polyvinyl alcohol (PVA), cellulose nanobers (CNFs), and polyaniline (PANI) are employed to enhance the bonding strength between the nanosheets.This effective approach signicantly enhances the mechanical properties of the sensing materials. 45Pressure sensing mechanism and evaluation mechanism of MXene material   undergoes deformation, leading to a change in the conductive path within the material.This change is then manifested as a variation in resistance that can be measured externally.The working mechanism of the piezoresistive sensor based on MXene material is depicted in Fig. 4. In this gure, R total represents the total resistance, while R 1 corresponds to the resistance of MXene nanosheets with initially small spacing.Under pressure, the resistance of R 1 remains nearly constant.On the other hand, R c represents the resistance between MXene sheets with initially larger spacing, which decreases as the distance between adjacent MXene sheets reduces due to the external load (Fig. 5a).Consequently, the internal resistance decreases, resulting in an overall increase in conductivity.The larger the distance D w between the two lamellae, the easier it is compressed, and the smaller the distance D n between the two lattices, the smaller the compression ratio. 46.1.2Capacitive pressure sensor.Capacitive pressure sensors detect external pressure by monitoring the change in capacitance when subjected to mechanical pressure.The sensing mechanism of a capacitive sensor is illustrated in the gure. 47To optimize the performance of capacitive sensors, various parameters are typically adjusted, such as the electrode area, the distance between the electrodes on both sides, and the dielectric constant of the material (Fig. 5b).The dielectric layer is sandwiched between two exible electrodes, and the size of the sensor changes when subjected to external forces.C represents the central component of the capacitor; R electrodes represent the residual resistance of the electrode; R dielectric represents the current leaking in the dielectric material; L is the inductance of the wire.MXene materials, owing to their high conductivity and exibility, can serve as electrodes in capacitive sensors or be utilized to fabricate microstructures that enable variable area modulation under pressure. 48.1.3Piezoelectric sensor.The piezoelectric pressure sensor is primarily based on the characteristics of piezoelectric materials.Under the inuence of external force, the material's internal charge becomes non-uniform, resulting in charge polarization.The charge polarization is eliminated upon external unloading.Studies have demonstrated that MXene exhibits a high orientation as a piezoelectric material with a non-centrosymmetric lattice structure.Fig. 5c presents a schematic representation of a single-layer Ti 3 C 2 T x piezoelectric device.This device achieves tensile and compressive strain of Ti 3 C 2 T x by bending the PET substrate outward and inward.The electrodes of the device are connected to an external circuit for the measurement and recording of piezoelectric signals.The red and purple arrows represent the current direction and polarity direction of Ti 3 C 2 T x under different states, respectively.49 3.2 Evaluation index 3.2.1 Sensitivity.The sensitivity of a exible sensor is the degree to which the sensor responds to a change in an external physical quantity.In exible sensors, sensitivity usually refers to the proportionality of the sensor's output signal relative to the change in the input signal.The sensitivity of a exible sensor is usually measured by its GF (Gauge factor), and in general, a larger GF value indicates a higher sensitivity of the sensor and a more sensitive response to external physical quantities.In previous reports, the emergence of nonlinear sensors i.e., sensors with varying sensitivity under different sizes of pressure operating ranges, which is related to the sensing material as well as the sensor structural design, can be realized through the selection of appropriate GF values for accurate monitoring and control of different application scenarios.
3.2.2Detection range.The detection range refers to the load range within which the sensor can reliably detect changes in electrical signals under normal working conditions.In practical sensor monitoring, selecting a sensor with high sensitivity within the appropriate detection range enables more accurate measurement.Conversely, operating the sensor outside the detection range can shorten its service life or cause damage.
3.2.3Linearity.Linearity refers to the degree of linearity in the actual relationship curve between the sensor input and output.Under specied conditions, linearity is determined by the maximum deviation between the sensor's calibration curve and the tted line, expressed as a percentage of the full-scale output.A smaller value indicates better linear characteristics, resulting in more accurate detection signals and facilitating subsequent signal processing.This allows for direct use in calibration, display, or control using simple algorithms or circuits.
3.2.4Response time.The response time is an indicator of the speed at which the electrical signal of the strain sensor changes when it is subjected to a load.The main factors that inuence the response time include the viscoelasticity of the sensing material and the stability of both the sensing material and the electrode.Typically, the response time ranges from tens of milliseconds to hundreds of milliseconds.However, in piezoresistive sensors, response time is longer compared to capacitive sensors due to the inuence of the internal conductive structure.
The performance of the sensor can be evaluated based on several factors.Apart from the aforementioned sensor evaluation indicators, exibility, hysteresis, accuracy, and durability are also crucial indicators for assessing sensor performance.4 Research progress of pressure sensor based on MXene material

Aerogel/hydrogel
Aerogel is a nanoscale porous solid material formed by the solgel method.This method replaces the liquid phase in the gel with gas through a specic drying process.With its threedimensional network structure lled with gas, aerogel exhibits high porosity, high specic surface area, low density, and low thermal conductivity.In recent years, researchers have incorporated various nanoparticles, conductive polymers, and carbon materials into the aerogel matrix, leveraging nanotechnology to produce composite aerogels for sensing devices.
However, a single MXene material tends to stack easily, and its low aspect ratio and weak gel ability make it difficult to form a continuous porous structure.To address this issue, Niu et al. 50eveloped an alkali-based polyacrylonitrile nanober (aPANF/ MX-rGA) aerogel with a three-dimensional interconnected porous structure.Polyacrylonitrile is prepared via electrospinning, and alkali treatment of the nanobers enhances the interaction between polyacrylonitrile PAN nano-bers and graphene oxide (GO).The alkali-treated polyacrylonitrile nanobers (aPANF) act as the scaffold for the aerogel network on the GO sheet, while the presence of MXene imparts excellent electrical conductivity to the aerogel.The resulting aerogel demonstrates high sensitivity (331 kPa −1 at 0-500 Pa and 126 kPa −1 at 500 Pa to 7.5 kPa), rapid response time (71 ms for load response, 15 ms for recovery response), and exceptional structural stability (17 000 compression cycles).This sensor exhibits the ability to detect weak signals from the body (Fig. 6a), such as pulse and heartbeat, with high sensitivity.
Bi et al. 51 fabricated a composite aerogel of AgNWs/MXene through directional freezing.The directional freezing apparatus comprises two materials with signicantly different thermal conductivities, aluminum and polyurethane, which generate horizontal and vertical temperature gradients during the freezing process.These temperature gradients facilitate the controlled growth of the AgNWs/Ti 3 C 2 T x aerogel, resulting in a layered structure where the AgNWs intertwine between the Ti 3 C 2 T x nanosheets to prevent stacking.The hydrogen bonding between AgNWs and Ti 3 C 2 T x is achieved using calcium alginate (CA), formed by introducing sodium alginate (SA) and calcium chloride (CaCl 2 ).The piezoresistive pressure sensor exhibits exceptional sensitivity (645.69 kPa −1 ), with a minimum detection limit of 1.25 Pa.It also offers a short response time of 60 ms and displays good bending performance within a bending angle range of 30°to 90°.Moreover, the sensor demonstrates high stability even aer undergoing more than 1000 compression cycles (Fig. 6b).This sensor shows high sensitivity to lowpressure stimuli, enabling the detection of impact pressure generated by falling water droplets and collisions.
MXene's limited interlayer spacing and tendency to selfstack limit the changes in electronic channels under external pressure, thus hindering the exploitation of its excellent surface metal conductivity.Cheng et al. 23 proposed a gas foaming method to construct MXene aerogel with adjustable layer spacing.Fig. 9a presents a ow diagram illustrating the preparation of MXene aerogel using hydrazine hydrate (N 2 H 4 ) gas foaming.Fig. 9b and c depict the components and manufacturing process of the sensor.Firstly, an MXene paperbased interdigital electrode is prepared via laser engraving, and a layer of polyether imide (PEI) is electrospuned onto the interdigital electrode as a diaphragm.Finally, the components are assembled using self-healing polyurethane (PU).The interlayer porosity (54.4%) of the MXene aerogels prepared by gas foaming is signicantly higher than that of the original structure (18.2%), resulting in high sensitivity (1799.5 kPa −1 ), a fast response time (11 ms), and good stability (>25 000 cycles).Aer experiencing mechanical damage, the carboxyl groups on the fractured surface are reconnected through hydrogen bonds.In the self-healing performance experiment, the sensor, which was cut in half by scissors, successfully reconnected and retained its initial sensing ability even aer ve recovery cycles while supporting a weight of 100 g (Fig. 6c).
Multi-parameter sensors can provide a more comprehensive and accurate description of the actual situation by monitoring multiple parameters simultaneously.Wu et al. 52 presented a dual-sensing MXene aerogel capable of temperature and pressure measurements.By incorporating poly(ethylene oxide) (PEO) semi-crystalline polymers with varying molecular weights between MXene sheets, the PEO polymer undergoes changes in response to ambient temperature, thereby adjusting the distance between MXene sheets and altering the aerogel's resistivity for temperature sensing.Simultaneously, external pressure causes the lamellar structure of the MXene aerogel to contract or even bond, creating additional conductive pathways and modifying the aerogel's resistivity for pressure sensing (Fig. 6d).The aerogel demonstrates a pressure sensitivity of up to 777 kPa −1 , with a detectable pressure limit of 0.05 Pa.Through the sensor's unique material selection and structural design, the thermopiezoresistive MXene/PEO aerogel enables the detection and differentiation of pressure and thermal stimuli within the physiological temperature range of the human body.
Hydrogel materials possess unique advantages in the realm of exible sensing due to their high biocompatibility, strong adhesion, and remarkable self-healing ability.Enhancing the conductivity of hydrogel materials has become pivotal for their application as exible sensors.
Feng et al. 53 proposed a technique called directional freezing to fabricate MXene conductive hydrogels (PMZn) with anisotropy and low-temperature tolerance.In this method, MXene nanosheets were combined with polyvinyl alcohol (PVA) and zinc sulfate (ZnSO 4 ) solutions and frozen at a low temperature.During the freezing process, the solvent solidied along the temperature gradient, squeezing other particles between the resulting ice-crystal columns.The ice crystal columns acted as orientation templates, creating an ordered internal orientation structure within the hydrogel.Subsequently, PMZn was immersed in glycerol to replace the solvent.Glycerol, forming hydrogen bonds with PVA and MXene nanosheets, replaced part of the water in PMZn, yielding PMZn-GL hydrogels.The hydrogel exhibited a tensile/compressive anisotropy ratio (dened as the ratio between the parallel and orthogonal directions) of approximately 2.2 and 1.36, respectively.The electrical conductivity of PMZn-GL, measured horizontally using an electrochemical workstation, reached 56 mS m −1 , surpassing that of the orthogonal direction.These experiments showcased that the directed arrangement of the polymer network conferred excellent mechanical and electrical properties to the hydrogels.In the antifreeze test experiment, the horizontal tensile curve of PMZn-GL at −25 °C resembled that at room temperature, while the fracture strain of PMZn decreased at low temperatures.Differential scanning calorimetry (DSC) testing revealed a crystallization peak of −36.3 °C for PMZn-GL, signicantly higher than that of PMZn (Fig. 6e).This nding indicated that PMZn-GL, obtained through glycerol replacement, exhibited enhanced resistance to low temperatures.

Ink printing
MXene, in comparison to other 2D materials, possesses a myriad of distinctive properties.These include metal-like electrical conductivity, superior dispersion, negative surface charge, and hydrophilicity.Such characteristics render it particularly well-suited as an ink for printing applications. 27nkjet printing provides a simple and cost-effective approach to fabricating wearable sensors.It allows for the deposition of desired patterns onto diverse substrates, including paper, thermoplastics, and glass.Saleh et al. 54 employed the inkjet printing technique to deposit three layers of MXene ink onto poly(3,4-vinyldioxythiophene)-polystyrene sulfonic acid (PEDOT:PSS) to create the sensing lm.The resulting lm exhibited an electrical conductivity of 162.2 ± 24.2 S cm −1 and achieved a maximum bending radius of 2 cm during compression strain testing.The lm demonstrated a consistent change in resistance even aer 1000 bending cycles.Moreover, when exposed to air at room temperature for a period of 50 days, the resistance of the printed lm only exhibited a slight increase (R/ R 0 = 1.19 ± 0.09), thereby highlighting its resistance to airinduced deterioration.This underscores the suitability of the printed lm for applications involving exposure to air (Fig. 7a).
In comparison to inkjet printing, screen printing utilizes high-viscosity ink that is passed through a template screen and It possesses the function and internal structure similar to natural plant bers while incorporating the advantages of cellulose bers and nanocellulose, including high degradability, excellent biocompatibility, low density, high strength, large aspect ratio, and signicant specic surface area.Chen et al. 56 isolated lignocellulose by removing impurities from pine powder using acetone and subsequently extracted total cellulose from lignocellulose via a sodium chlorite (NaClO 2 )/acetic acid (CH 3 COOH) solution.The obtained cellulose was further modied with sulfamic acid and treated with a NaOH solution to obtain HCNF (hydroxylated cellulose nanober).MXene/ HCNF (MH) inks were prepared by blending MXene and HCNF at varying mass fractions.The rapid decrease in the contact angle value of MH ink on cellulose paper demonstrates the excellent printability of MH ink.MH inks display typical non-Newtonian shear-thinning behavior, ensuring the maintenance of the structure and shape of the printed product, which exhibits consistent resistance signals over 101% strain cycles.The resistance of the printed product exhibits a continuous variation trend with changes in ambient temperature and humidity, and the strain sensing response remains almost constant aer 3 months, indicating the stability of MH ink (Fig. 7c).3D printing, also known as additive manufacturing (AM), is a rapid process for constructing 3D objects by printing layer by layer, offering high degrees of freedom and short lead times.Li et al. 57 utilized adhesive jet (BJ) printing technology to combine polyvinyl alcohol (PVOH) with Ti 3 C 2 T x material, which is easily soluble in water and possesses excellent biocompatibility.They successfully prepared a 3D/exible MXene composite material measuring 4 cm × 1 cm × 1 mm.In the tensile test, the sample exhibited up to 250% deformation while maintaining stable conductivity at 50% deformation.The printed material demonstrated a linear sensitivity ranging from 0% to 80% strain, with a sensitivity value of 1.65 ± 0.16 over three repeated cycles (Fig. 7d).

Film/electronic skin
The electronic skin, which mimics the perception function of human skin, is a novel type of thin, so, and exible lm sensor.Yan et al. 58 reported a piezoresistive sensing lm with a bionic structure inspired by the ginkgo biloba leaf (Fig. 8a).The exible sensor reproduced the microstructure of the ginkgo biloba leaf surface onto a polydimethylsiloxane (PDMS) lm through molding.MXene nanosheets were then sprayed onto the lm's surface to create a sensing layer.A layer of PVA ber was inserted between the interdigital electrode and the lm using electrospinning.The bionic thin lm sensor exhibited a sensitivity of 403 kPa −1 , a response time of 99.3 ms, a low detectable pressure limit (0.88 Pa), and demonstrated continuous stability over 12 000 load-unload cycles.
Similar to conductive enhancers such as graphene and carbon nanotubes, MXene can also induce the formation of piezoelectric phases, facilitate strong interfacial coupling effects, and generate piezoelectric responses.Zhao et al. 59 proposed a highly sensitive piezoelectric sensor based on a mixed lm of MXene and polyvinylidene uoride (PVDF) by investigating the piezoelectric properties and sensitivity of MXene-modulated PVDF.The mixed lm was fabricated through electrospinning, hot pressing, and roller pressing.To enhance the sensing performance of the sensor, Au electrodes were deposited on the lm's surface using a corona polarization device, and the PVDF hybrid lm was encapsulated with PET lm.At a low loading level of 0.4 wt%, the piezoelectric coefficient d 33 of the MXene/PVDF hybrid lm reached a peak value of 43 pC N −1 .Simultaneously, the addition of MXene nanosheets contributed to improving the mechanical properties of PVDF.The MXene/PVDF hybrid membrane sensor exhibited a voltage sensitivity of 0.0048 V N −1 , which was twice that of the PVDFbased sensor (Fig. 8b).
The accurate recognition of physiological and physical signals is crucial for the performance of exible sensors.However, improving the stability of exible sensors in harsh

Review
RSC Advances environments remains a signicant challenge.Zhao et al. 60 developed a highly stable electronic skin, the MXene/PVA hybrid lm, by synergistically binding strong hydrogen bonds between MXene and polyvinyl alcohol (PVA).The mixed membrane was fabricated by vacuum ltration of the MXene/ PVA mixture through a cellulose membrane with a pore size of 0.22 nm.Through a 7 day soaking test in different solutions to assess its stability in various environments, the MXene/PVA mixed lm exhibited weight loss of 1.2 ± 0.2 wt% in water, 4.0 ± 0.2 wt% in acid, and 4.5 ± 0.3 wt% in alkali solution, which was 22-83 times lower than that of the pure MXene membrane.Even aer 24 hours of soaking, the mixed lm maintained an elastic modulus of over 95%.In cell compatibility experiments, the survival rate of human umbilical vein endothelial cells aer 7 days was 99.8 ± 0.9% (Fig. 8c and d), and the mixed membrane enabled stable in vivo heartbeat monitoring in anesthetized mice.

Fibrous fabric
The exible device, comprised of bers and exible fabric, serves as a fundamental solution to address challenges associated with wearing comfort, wash resistance, and adherence to the human body.Simultaneously, it aligns with the principles of green environmental protection.The inherent roughness of cotton bers promotes the adhesion of nanosheets and imparts excellent air permeability.Liu et al. 61 achieved the fabrication of MXene cotton fabric by coating Ti 3 C 2 T x nanosheets onto cotton bers, resulting in strong bonding and consequently yielding a fabric with remarkable air permeability (972.2 mm s −1 ) and moisture permeability (227.92g m −2 ).The sensor demonstrated exceptional performance, including high sensitivity (7.67), rapid response and recovery time (35 ms), outstanding stability (over 2000 cycles), and a wide sensing range (Fig. 9a and b).Seyedin et al. 62 fabricated MXene/polyurethane (PU) skin and PU core bers through coaxial wet spinning.In comparison to non-coaxial composite bers, these bers exhibited enhanced stability when subjected to cyclic strains of varying magnitudes.The MXene/PU composite ber demonstrated a strain sensing capacity of 152% and achieved a sensitivity (GF) of approximately 12 900.Knitted fabrics constructed with this ber showcased a strain sensing ability of up to 200% and maintained exceptional stability even aer undergoing 1000 cycles of stretch and release deformation (Fig. 9c).To enable tracking of diverse elbow movements, the MXene/PU ber was woven into an elbow sheath using a knitting machine.Duan et al. 25 devised a three-layer cored shell structure to create a smart ber.The smart ber, known as SPMP ber, comprises four layers: a spandex core, a PVA layer, an MXene layer, and a PDMS layer.The SPMP bers manifest a sensitivity (GF) of 10.3 within the strain range of 40% to 80%.When compared to unencapsulated bers, the resistance variation of SPMP bers remained within ±0.6% aer being immersed in water, sweat, and saline solutions for 15 days, signifying exceptional washability and water resistance.To demonstrate the capabilities of the SPMP ber, the researchers constructed a waterproof hybrid electronic system utilizing machine learning techniques.They deployed gloves as carriers for the exible bers, integrated ber sensors, and processing centers onto a exible printed circuit board (FPCB) using integrated packaging technology.Through this setup, they successfully accomplished underwater piano playing and recognized 20 gestures for remote-controlled robot hands with an accuracy rate of 98.1% (Fig. 9d).
We have summarized the preparation method and performance index for MXene-based sensing materials and presented them in Table 1.

Summary and prospect
MXene materials exhibit excellent application potential in the realm of exible pressure sensing owing to their remarkable electrical conductivity, mechanical properties, unique layered structure, and abundant functional groups on the surface.By integrating MXenes with other materials, a novel structural system can be developed to enhance the synergistic effects of MXene materials, leading to the creation of exible sensors characterized by high sensitivity and a broad detection range.This paper presents an overview of research achievements in MXene-based exible sensing across various domains, including hydrogel/aerogel, ink printing, paper-based lm, and ber fabric.The preparation strategies, sensing properties, and applications of these sensors in human body detection are also discussed.Despite the exceptional performance and potential displayed by MXene-based exible pressure sensors, there still exist numerous challenges and issues that necessitate further exploration and resolution.
Assuredly, stability performance is crucial for exible sensors, the easy oxidation of MXene material is still a problem to be solved, the sensor will inevitably oxidize during the working process, which will greatly reduce the stability and service life of the sensing device.Therefore how to enhance the service life of MXene-based exible sensors is an issue worthy of further research.Secondly, among the MXene-based exible sensors reported now, due to the limitations of material properties and manufacturing processes, they have not yet been able to meet the demands of some specic applications, such as in high temperature, high humidity, or strong corrosive environments, the performance of MXene exible sensors will be affected, which restricts the application in these environments.Packaging plays a critical role in the functionality of exible sensors, particularly in wearable sensor applications.Appropriate packaging techniques allow subjects to wear the device for extended durations while minimizing signal dri and noise.Hence, it becomes essential to identify suitable packaging methods that enable the acquisition of more accurate human data and enhance wearer comfort.Finally, the mechanical forces applied to sensors typically involve a combination of pressure, tension, shear, and torsion forces.Decoupling these mixed forces becomes particularly crucial for applications such as gesture recognition, robot control, and prosthetics.To achieve this, sophisticated structural designs are needed to differentiate between different types of mechanical stimuli and assign them to the corresponding sensors.This enables independent detection of deformations caused by distinct forces.

Fig. 2
Fig. 2 Pathways to synthesize MXene flakes using a top-down etching method.Copyright© 2020, John Wiley and Sons.

3. 1
Sensing mechanism 3.1.1Piezoresistive pressure sensor.When an external load is applied to the piezoresistive sensor, the sensing material

Fig. 3
Fig. 3 (a) Schematic and SEM images of Ti 3 C 2 T x -based field effect tubes.Copyright© 2016, John Wiley and Sons.(b) Schematic representation of 2D transition metal nitrides by elevated temperature ammoniation.Copyright© 2016, Royal Society of Chemistry.

Fig. 5
Fig. 5 (a) The working mechanism of piezoresistive sensor based on MXene material.Copyright© 2017, Springer Nature.(b) Schematic diagram of flexible capacitive sensor.Copyright© 2021, John Wiley and Sons.(c) Working mechanism of a single layer Ti 3 C 2 T x MXene piezoelectric sensor Copyright© 2021, Elsevier.

Fig. 6
Fig. 6 (a) Schematic diagram of the preparation process of aPANF/MX-rGA aerogel.Copyright© 2021, John Wiley and Sons.(b) Schematic diagram of preparation of AgNWs/Ti 3 C 2 T x aerogel by directional freezing.Copyright© 2020, Royal Society of Chemistry.(c) Flexible pressure sensor with maximum electronic channel and self-healing prepared by gas foaming process.Copyright© 2023, American Chemical Society.(d) Schematic illustration of pyroresistive and piezoresistive mechanism for MXene/PEO aerogel.Copyright© 2022, Royal Society of Chemistry.(e) The synthesis procedures of PMZn-GL hydrogels and further applications in wearable flexible sensors and 3D sensor arrays.Copyright© 2021, John Wiley and Sons.

Fig. 7
Fig. 7 (a) Schematic illustration of three MXene electrodes printed on flexible self-standing PEDOT substrates.Copyright© 2020, IOP Publishing.(b) Screen-printing flexible MXene patterns for EMI, Joule heater and piezoresistive sensor devices.Copyright© 2022, John Wiley and Sons.(c) Schematic representations of sulfated HCNF with the "core-shell" structure.Copyright© 2021, American Chemical Society.(d) MXene inks for 3D printing vary in contact angle under different conditions.Copyright© 2022, Royal Society of Chemistry.

Fig. 8
Fig. 8 (a) Manufacturing technology of maple leaf bionic piezoresistive sensor.Copyright© 2022, Elsevier.(b) Structure diagram of piezoelectric PVDF hybrid film.Copyright© 2021, Elsevier.(c) Diagram of pressure sensors attached to the wall of the heart and stomach in mice.Copyright© 2021, Elsevier.(d) The schematic illustration of the skin-like PVA/MXene hybrid thin-film with crosslinked structures.Copyright© 2021, Elsevier.

Table 1
Performance data based on MXene pressure sensors